Many different types of power supplies are available. One type of power supply is referred to as a direct current (DC) power supply because it generally provides an output in the form of a current that is directly related to a DC voltage. One particular type of DC power supply is used to emulate the performance of a solar array. Such a power supply is sometimes referred to as a solar array simulator (SAS).
A solar array simulator is a specialized DC power supply that acts to simulate the static current/voltage (I/V) characteristics of silicon or gallium arsenide solar panels or arrays of solar panels. The arrays being simulated are generally, although not exclusively, of the type used in spacecraft such as communications satellites. To the extent possible, these power supplies also attempt to simulate dynamic behavior. Dynamic behavior is exhibited in two distinct dimensions. The first of these relates to the dynamic response of the solar array to changing load conditions. The second relates to the solar array's dynamic response to changing illumination which typically occurs in response to eclipse events or as the array's orientation towards the sun varies.
The I/V characteristics of a solar array may be simulated by means of one of several available mathematical models. One such model is known by those skilled in the art as the exponential model. The exponential model describes the array output voltage as a function of load current. A numerical algorithm derived from this model may be used to define a voltage-input, current-output basis for simulation, i.e., operation as a current source. In purely static or low-bandwidth dynamic implementations, voltage supplied to the load is measured and used in a numerical algorithm to extract a controlled current value corresponding to the array's ability to source current at the measured voltage value. The power supply is operated as a constant current (CC) source and is programmed to the extracted value thus simulating the I/V characteristic of the solar array. Within resolution limits defined by the algorithm and the hardware implementation, each measured voltage value has a corresponding unique current value. A number of these ordered pairs may be used to define a current versus voltage curve which characterizes the particular array's characteristic for a given illumination. Changes in the array's design or configuration, changes to the semiconductor properties of silicon or gallium arsenide cells used in the array, changes in the connections between the individual cells making up the array, and changes in illumination or temperature are amongst a variety of factors that change the I/V characteristic curve.
Bandwidth in the two dimensions of dynamic behavior noted above may be improved in the first case by increasing the speed with which table entries may be retrieved in response to changing load voltage and in the second case by pre-calculating sets of ordered I/V pairs and assembling these into look-up tables which may be rapidly switched in response to changes in illumination. Both may be thought of as improvements in bandwidth.
With regard to the dynamic response of the solar array to changing load conditions, bandwidth may be improved by increasing the speed with which table entries are applied in response to changing load voltage. This can be accomplished by, for example, increasing the speed with which table entries are searched in response to changing load conditions.
With regard to the solar array's dynamic response to changing illumination, it is desirable to accurately simulate transitions into and out of an eclipse because the eclipse causes changes in illumination, which will in turn change the I/V characteristic of the solar array. This gives rise to a desire to have the ability to rapidly switch tables. Some satellites are configured to keep the solar arrays pointed towards the sun at all times. For these, eclipse occurs when the satellite enters the Earth's shadow. The speed with which eclipse occurs along with the perceived need to accurately simulate transitional conditions determines table update rates. Other satellites are configured to operate while spinning. With these, eclipse occurs repetitively as the satellite spins and may exhibit periods as short as three seconds or less. Under these conditions, table update rates may be quite high particularly if there is a desire to accurately simulate transitional conditions between full illumination and total eclipse.
An additional factor to be considered for simulations of complex solar arrays on high-power satellites involves solar arrays having numerous panels. These complex arrays typically comprise numerous individual panels which may have varying orientations with respect to the sun, particularly in the case where the satellite is designed to spin. For such configurations, a system of fifty or more separate SAS “channels” may be required, each of which should be capable of rapidly switching tables in tight synchronization. Moreover, if a high accuracy simulation of transitional conditions is desired, it may be necessary to rapidly cycle through a large number of tables on each channel. Because of the varying orientation of individual panels, each channel may have a similar or identical set of tables, but also a different “phase” relationship with respect to the satellite and its source of illumination. A typical scenario might involve 72 channels with 36 tables each (corresponding to 10 degree rotational resolution) and a spin period of 10 seconds. A system designed to support this application would need to provide table changes at an aggregate rate of ˜260 tables per second. Assuming also that each table consists of 4096 entries (a typical value), the aggregate data rate is in excess of one million entries per second. The equations and numerical methods used for defining I/V relationships are such that real-time calculation of table entries at this rate entails an extreme computational load.
Therefore, it would be desirable to simulate dynamic behavior as it relates to the dynamic response of the solar array to changing load conditions and as it relates to the response of the solar array to changing illumination.